public class Practice {
    public static void main(String[] args) {
        String s1,s2,s3="Hello World";
        s1=s3.toLowerCase();
        s2=s3.toUpperCase();
        System.out.println("一开始是："+s3);
        System.out.println("s1是："+s1);
        System.out.println("s2是："+s2);
        s3=s1.concat(s2);
        System.out.println("s3是："+s3);
    }
}